1. Field of the Invention
The present invention relates to magnetic sensing elements which utilize a tunneling effect and which are to be mounted on magnetic reproducing devices, such as hard disk drives, or other magnetic sensing devices. More particularly, the invention relates to a tunneling magnetic sensing element which has a high rate of change in resistance (ΔR/R) without greatly increasing the magnetostriction λ of its free magnetic layer, the element thus having excellent magnetic sensitivity and stability, and a method for manufacturing the same.
2. Description of the Related Art
In a tunneling magnetic sensing element (tunneling magnetoresistance element), the change in resistance is caused by a tunneling effect. When the magnetization direction of a pinned magnetic layer is antiparallel to the magnetization direction of a free magnetic layer, a tunnel current does not easily flow through an insulating barrier layer (tunnel barrier layer) provided between the pinned magnetic layer and the free magnetic layer, and the resistance is at a maximum. On the other hand, when the magnetization direction of the pinned magnetic layer is parallel to the magnetization direction of the free magnetic layer, the tunnel current flows easily, and the resistance is at a minimum.
By use of the principle described above, a change in electrical resistance caused by a variation in the magnetization of the free magnetic layer under an influence of an external magnetic field is captured as a change in voltage, and thus a leakage magnetic field from a recording medium is detected.
Japanese Unexamined Patent Application Publication No. 2001-223412 (Patent Document 1) and Japanese Unexamined Patent Application Publication No. 2006-196745 (Patent Document 2) each describe a magnetoresistance element in which a film that prevents interdiffusion is disposed between a magnetic layer and a protective film.
Japanese Unexamined Patent Application Publication No. 2000-106462 (Patent Document 3) describes a tunneling magnetic sensing element.
In tunneling magnetic sensing elements, one of the problems to be solved is increasing detection sensitivity by achieving a high rate of change in resistance (ΔR/R) to improve the characteristics of read heads. In order to increase the rate of change in resistance (ΔR/R) of a tunneling magnetic sensing element, it is known to be effective to change the composition of the free magnetic layer, for example, by using a material having high spin polarizability at the interface with the insulating barrier layer.
When the iron (Fe) content at the interface with the insulating barrier layer is increased, spin polarizability can be increased and the rate of change in resistance (ΔR/R) can be increased. However, if the Fe content is high, the magnetostriction λ of the free magnetic layer has a large positive value, which causes noise in read heads, resulting in a decrease in stability of read heads.
Besides the method described above, the rate of change in resistance (ΔR/R) can be increased by changing the compositions of the free magnetic layer and the pinned magnetic layer. However, if the compositions of the free magnetic layer and the pinned magnetic layer are changed, other magnetic properties are changed. Therefore, it is desirable to obtain a high rate of change in resistance (ΔR/R) without changing the compositions or thickness of the free magnetic layer and the pinned magnetic layer.
In tunneling magnetic sensing elements, it is important to optimize the structures of the insulating barrier layer and the free magnetic layer in order to increase the rate of change in resistance (ΔR/R). For example, when an insulating barrier layer is composed of magnesium oxide (Mg—O), it is known that, in order to increase the rate of change in resistance (ΔR/R) of the tunneling magnetic sensing element, it is preferable that the free magnetic layer in contact with the insulating barrier layer has a body-centered cubic (bcc) structure.
In such a case, when the protective layer for preventing oxidation disposed on the free magnetic layer is composed of tantalum (Ta), for example, in the heat treatment step in the fabrication process, Ta of the protective layer diffuses into the free magnetic layer and further into the insulating barrier layer, and the free magnetic layer and the insulating barrier layer are inhibited from being crystallized. As a result, strain occurs in the bcc structure of the free magnetic layer or the insulating barrier layer, and it is not possible to obtain a high rate of change in resistance (ΔR/R).
In the giant magnetoresistance element described in Patent Document 1, in order to prevent interdiffusion between a protective layer and a magnetic layer, an interdiffusion-preventing film composed of a high-melting transition metal, such as Ta or Ti, is provided. Thereby, interdiffusion is prevented between the protective layer composed of ruthenium (Ru), platinum (Pt), or the like and the magnetic layer.
Patent Document 2 describes that by disposing an intermediate layer composed of platinum-manganese (PtMn) between a protective layer composed of Ta and a free magnetic layer, interdiffusion between the free magnetic layer and the protective layer can be prevented.
However, neither Patent Document 1 nor Patent Document 2 describes a tunneling magnetic sensing element. That is, neither of them discloses an optimum structure of a protective layer on a free magnetic layer in a tunneling magnetic sensing element.
Furthermore, Patent Document 3 describes the invention relating to a tunneling magnetic sensing element and discloses use of Ta, Ti, or the like for a protective layer. However, Patent Document 3 does not describe at all that by optimizing the protective layer on the free magnetic layer, the rate of change in resistance (ΔR/R) is increased. In Patent Document 3, Ta is used as the protective layer in many examples. However, in such a case, as described above, because of diffusion of Ta into the free magnetic layer and the insulating barrier layer, the rate of change in resistance (ΔR/R) is decreased, which is a problem.